BALTIMORE (AP) — A school bus was blocks away from its first stop Tuesday morning when it rear-ended a car and then ricocheted off a roadside pillar into an oncoming commuter bus. The pre-dawn accident killed at least six people and injured 10, authorities said.

There were virtually no skid marks at the crash scene, suggesting that the brakes of the school bus were not forcefully applied, and leading to what Baltimore Police spokesman T.J. Smith called a working theory that the driver had suffered some sort of medical emergency.

The school bus driver was killed, along with at least five people on the Maryland Transit Administration bus, Smith said.

The only other occupant of the school bus, an aide, was taken to a hospital, as were the car driver and eight people from the commuter bus, Smith told a news conference.

He said one survivor was in critical condition, one was in serious condition and eight had injuries that were not considered serious.

A National Transportation Safety Board team began examining the vehicles and documenting the scene early Tuesday afternoon, spokesman Keith Holloway said.

Baltimore Police Commissioner Kevin Davis said riders on the No. 10 MTA bus were likely going to work on a route from Dundalk, a largely blue-collar community southeast of Baltimore, toward Catonsville, a western suburb.

“They’re on their way to make a living, they’re on their way to the job and they’re on their way to support their families,” Davis said, “Our hearts and prayers go out to them, to their families, to their co-workers as well.”

The school bus first hit a silver Ford Mustang, crushing its rear and forcing its nose into the pavement. Then it hit a pillar at a cemetery entrance hundreds of feet down the street. Veering across the center line, the yellow school bus slammed into the front driver side of the MTA bus, ending about 100 yards from the pillar.

The school bus raked the side of the commuter bus, ending with its front end buried toward the back of the MTA vehicle on Frederick Avenue near Loudon Park Cemetery.

The wreck peeled back metal and exposed a tangle of metal and human limbs, according to a passing motorist who stopped to help, Michael Feldman. He told WJZ-TV that the MTA driver appeared to be in very bad shape; authorities have not said if that driver survived.

The Mustang driver, Shawn Braxton, told reporters he was driving to work when the bus hit his car. He said he only remembers hearing the impact.

“Physically, I’m bruised, banged up,” Braxton said. “Mentally, it’s going to take a while to get myself together.”

The school bus was contracted from Baltimore-based AA Affordable Transportation and served 18 elementary students, the city’s public school system said in a statement.

Federal records show the company had no violations or other crashes reported in the two years ending this past September. In 2012, an AA Affordable school bus was involved in a crash that injured at least 13 people, including students, when it collided with a minivan, the Baltimore Sun reported.

One of the company’s drivers, Chernell Burrows, told reporters that the late driver was kind and dependable.

UPDATE: There were no children on the school bus, however, there are six fatalities. We will brief soon.

Authorities had not reported any more victims by mid-afternoon. Baltimore Fire Chief Niles Ford told a morning news conference that there were parts of the MTA bus that firefighters had not yet reached.

He said firefighters had to enter the school bus from the rear and cut out the seats to reach the driver and aide.

Some of the survivors suffered injuries to their faces and spines, said Dr. Deborah Stein at the University of University of Maryland Medical Center’s shock trauma unit in Baltimore, where five of the injured were taken. Two were taken to St. Agnes Hospital, according to a statement from St. Agnes Healthcare.

The Rev. Mike Murphy said the crash sounded like “one loud thump” from his room in the rectory of nearby St. Joseph’s Monastery, a Catholic facility.

Murphy said the busy thoroughfare “gets kind of crazy at times.”

Doreen Downs, who lives nearby, heard the crash and saw the wreckage.

“It’s just horrible,” she said.

Smith characterized it as an accident investigation, not a crime-scene investigation, despite the presence of homicide detectives. Smith said they were called because they are accustomed to conducting death investigations.